boost-react-native-bundle

<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Constructor Forwarding</title> <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../move.html" title="Chapter 19. Boost.Move"> <link rel="prev" href="move_and_containers.html" title="Containers and move semantics"> <link rel="next" href="move_return.html" title="Implicit Move when returning a local object"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> <td align="center"><a href="../../../index.html">Home</a></td> <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="move_and_containers.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../move.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="move_return.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="move.construct_forwarding"></a><a class="link" href="construct_forwarding.html" title="Constructor Forwarding">Constructor Forwarding</a> </h2></div></div></div> Consider writing a generic factory function that returns an object for a newly constructed generic type. Factory functions such as this are valuable for encapsulating and localizing the allocation of resources. Obviously, the factory function must accept exactly the same sets of arguments as the constructors of the type of objects constructed: <pre class="programlisting">template<class T> T* factory_new() { return new T(); } template<class T> T* factory_new(a1) { return new T(a1); } template<class T> T* factory_new(a1, a2) { return new T(a1, a2); } </pre> Unfortunately, in C++03 the much bigger issue with this approach is that the N-argument case would require 2^N overloads, immediately discounting this as a general solution. Fortunately, most constructors take arguments by value, by const-reference or by rvalue reference. If these limitations are accepted, the forwarding emulation of a N-argument case requires just N overloads. This library makes this emulation easy with the help of <code class="computeroutput">BOOST_FWD_REF</code> and <code class="computeroutput">boost::forward</code>: <pre class="programlisting">#include <boost/move/utility_core.hpp> #include <iostream> class copyable_only_tester { public: copyable_only_tester() { std::cout << "copyable_only_tester()" << std::endl; } copyable_only_tester(const copyable_only_tester&) { std::cout << "copyable_only_tester(const copyable_only_tester&)" << std::endl; } copyable_only_tester(int) { std::cout << "copyable_only_tester(int)" << std::endl; } copyable_only_tester(int, double) { std::cout << "copyable_only_tester(int, double)" << std::endl; } }; class copyable_movable_tester { // move semantics BOOST_COPYABLE_AND_MOVABLE(copyable_movable_tester) public: copyable_movable_tester() { std::cout << "copyable_movable_tester()" << std::endl; } copyable_movable_tester(int) { std::cout << "copyable_movable_tester(int)" << std::endl; } copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester)) { std::cout << "copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester))" << std::endl; } copyable_movable_tester(const copyable_movable_tester &) { std::cout << "copyable_movable_tester(const copyable_movable_tester &)" << std::endl; } copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester), BOOST_RV_REF(copyable_movable_tester)) { std::cout << "copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester), BOOST_RV_REF(copyable_movable_tester))" << std::endl; } copyable_movable_tester &operator=(BOOST_RV_REF(copyable_movable_tester)) { std::cout << "copyable_movable_tester & operator=(BOOST_RV_REF(copyable_movable_tester))" << std::endl; return *this; } copyable_movable_tester &operator=(BOOST_COPY_ASSIGN_REF(copyable_movable_tester)) { std::cout << "copyable_movable_tester & operator=(BOOST_COPY_ASSIGN_REF(copyable_movable_tester))" << std::endl; return *this; } }; //1 argument template<class MaybeMovable, class MaybeRv> void function_construct(BOOST_FWD_REF(MaybeRv) x) { MaybeMovable m(boost::forward<MaybeRv>(x)); } //2 argument template<class MaybeMovable, class MaybeRv, class MaybeRv2> void function_construct(BOOST_FWD_REF(MaybeRv) x, BOOST_FWD_REF(MaybeRv2) x2) { MaybeMovable m(boost::forward<MaybeRv>(x), boost::forward<MaybeRv2>(x2)); } int main() { copyable_movable_tester m; //move constructor function_construct<copyable_movable_tester>(boost::move(m)); //copy constructor function_construct<copyable_movable_tester>(copyable_movable_tester()); //two rvalue constructor function_construct<copyable_movable_tester>(boost::move(m), boost::move(m)); copyable_only_tester nm; //copy constructor (copyable_only_tester has no move ctor.) function_construct<copyable_only_tester>(boost::move(nm)); //copy constructor function_construct<copyable_only_tester>(nm); //int constructor function_construct<copyable_only_tester>(int(0)); //int, double constructor function_construct<copyable_only_tester>(int(0), double(0.0)); //Output is: //copyable_movable_tester() //copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester)) //copyable_movable_tester() //copyable_movable_tester(const copyable_movable_tester &) //copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester), BOOST_RV_REF(copyable_movable_tester)) //copyable_only_tester() //copyable_only_tester(const copyable_only_tester&) //copyable_only_tester(const copyable_only_tester&) //copyable_only_tester(int) //copyable_only_tester(int, double) return 0; } </pre> Constructor forwarding comes in handy to implement placement insertion in containers with just N overloads if the implementor accepts the limitations of this type of forwarding for C++03 compilers. In compilers with rvalue references perfect forwarding is achieved. </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> <td align="right"><div class="copyright-footer">Copyright © 2008-2014 Ion Gaztanaga Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="move_and_containers.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../move.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="move_return.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>